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J. Bacteriol., 06 1997, 3711-3720, Vol 179, No. 11
K Mathee, CJ McPherson and DE Ohman
Pseudomonas aeruginosa strains associated with cystic fibrosis are often
mucoid due to the copious production of alginate, an exopolysaccharide and
virulence factor. Alginate gene expression is transcriptionally controlled
by a gene cluster at 68 min on the chromosome: algT (algU)-mucA-mucB
(algN)-mucC (algM)-mucD (algY). The algT gene encodes a 22-kDa alternative
sigma factor (sigma22) that autoregulates its own promoter (PalgT) as well
as the promoters of algR, algB, and algD. The other genes in the algT
cluster appear to regulate the expression or activity of sigma22. The goal
of this study was to better understand the functional interactions between
sigma22 and its antagonist regulators during alginate production. Nonmucoid
strain PAO1 was made to overproduce alginate (indicating high algD promoter
activity) through increasing sigma22 in the cell by introducing a plasmid
clone containing algT from mucA22(Def) strain FRD1. However, the bacterial
cells remained nonmucoid if the transcriptionally coupled mucB on the clone
remained intact. This suggested that a stoichiometric relationship between
sigma22 and MucB may be required to control sigma factor activity. When the
transcription and translational initiation of algT were measured with lacZ
fusions, alginate production correlated with only about a 1.2- to 1.7-fold
increase in algT-lacZ activity, respectively. An algR-lacZ transcriptional
fusion showed a 2.8-fold increase in transcription with alginate production
under the same conditions. A Western blot analysis of total cell extracts
showed that sigma22 was approximately 10-fold higher in strains that
overproduced alginate, even though algT expression increased less than
2-fold. This suggested that a post- transcriptional mechanism may exist to
destabilize sigma22 in order to control certain sigma22-dependent promoters
like algD. By Western blotting and phoA fusion analyses, the MucB
antagonist of sigma22 was found to localize to the periplasm of the cell.
Similar experiments suggest that MucA localizes to the inner membrane via
one transmembrane domain with amino- and carboxy-terminal domains in the
cytoplasm and periplasm, respectively. These data were used to propose a
model in which MucB-MucA-sigma22 interact via an inner membrane complex
that controls the stability of sigma22 protein in order to control alginate
biosynthesis.
Copyright © 1997, American Society for Microbiology
Posttranslational control of the algT (algU)-encoded sigma22 for expression of the alginate regulon in Pseudomonas aeruginosa and localization of its antagonist proteins MucA and MucB (AlgN)
Department of Microbiology and Immunology, University of Tennessee, Veterans Affairs Medical Center, Memphis 38163, USA.
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